x86/fpu: Fix i486 + no387 boot crash by only saving FPU registers on context switch if there is an FPU

Sebastian Andrzej Siewior · Ingo Molnar · commit 2224d6165281 · 2018-10-17T12:30:38.000+02:00
Booting an i486 with "no387 nofxsr" ends with with the following crash: math_emulate: 0060:c101987d Kernel panic - not syncing: Math emulation needed in kernel on the first context switch in user land. The reason is that copy_fpregs_to_fpstate() tries FNSAVE which does not work as the FPU is turned off. This bug was introduced in: f1c8cd0 ("x86/fpu: Change fpu->fpregs_active users to fpu->fpstate_active") Add a check for X86_FEATURE_FPU before trying to save FPU registers (we have such a check in switch_fpu_finish() already). Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Reviewed-by: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: stable@vger.kernel.org Fixes: f1c8cd0 ("x86/fpu: Change fpu->fpregs_active users to fpu->fpstate_active") Link: http://lkml.kernel.org/r/20181016202525.29437-4-bigeasy@linutronix.de Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
@@ -528,7 +528,7 @@ static inline void fpregs_activate(struct fpu *fpu)
 static inline void
 switch_fpu_prepare(struct fpu *old_fpu, int cpu)
 {
-	if (old_fpu->initialized) {
+	if (static_cpu_has(X86_FEATURE_FPU) && old_fpu->initialized) {
 		if (!copy_fpregs_to_fpstate(old_fpu))
 			old_fpu->last_cpu = -1;
 		else

Original file line number	Diff line number	Diff line change
`@@ -528,7 +528,7 @@ static inline void fpregs_activate(struct fpu *fpu)`
`528`	`528`	`static inline void`
`529`	`529`	`switch_fpu_prepare(struct fpu *old_fpu, int cpu)`
`530`	`530`	`{`
`531`		`- if (old_fpu->initialized) {`
	`531`	`+ if (static_cpu_has(X86_FEATURE_FPU) && old_fpu->initialized) {`
`532`	`532`	`if (!copy_fpregs_to_fpstate(old_fpu))`
`533`	`533`	`old_fpu->last_cpu = -1;`
`534`	`534`	`else`